With clean, freshly made polyamides which have not been subjected to auxiliary processing, processing to products with quality features such as possessed by new goods is quite easily possible. However, if articles of daily use which have been used for many years or products from a collection of plastics are to be recycled, this can only be carried out with a highly technical, energy-consuming and expensive effort.
Here, cleaning and sorting stages are necessary to obtain sorted material, because plastic products are always matched to the field of application and therefore contain additives, colorants, stabilisers, glass fibre, etc. specific to the manufacturer or manufacturing process and which make processing difficult. Consequently, with the economical recycling of polyamide waste, cracking into the monomers is essential if a product is to be produced which is indiscernible from products manufactured from monomers through the synthesis route.
Caprolactam is the monomer which is used to manufacture Polyamide 6 from which parts for vehicles and injection moulded parts with glass fibre and other additives are made and from which fibres for yarn are spun that are used in carpets and carpet floor covering materials or other objects used in daily life.
A large part of this material is fed into plastics circulation after the intended use of the products and is reused via plastics recycling. Since the Polyamide 6 products are usually not the sole material containing amide groups, but rather polyamides are also present which consist of various diacids and diamines, identification and sorting into the individual polyamide product groups must take place. If this separation does not take place, then substantial problems due to mixed reactions of the various monomer groups, yield losses due to byproducts and insufficient quality must be expected with the monomer obtained. The major share of the Polyamide 6 plastic material, for example, goes into the manufacture of fibres and in particular into applications in floor coverings. The Polyamide 6 is spun to form fibres which are needled in carpet factories to form a backing fabric, e.g. polypropylene fleece, and are processed to looped or velour fabrics depending on the type of carpet. After needling of the yarn, which is pulled off bobbins, onto the backing fabric, the pile is fixed by an adhesive layer onto which a polymer foam backing is then applied which is filled with a filling material, such as chalk. One purpose which the foam backing fulfils is to efficiently deaden footsteps and also to produce the necessary weight so that the carpet lies flat on the floor without gluing.
The processing of Polyamide 6 waste after the intended use has been described in various patents, such as for example DE 19719734, EP 0681896, U.S. Pat. No. 5,598,980, DE 2416573 and DE 2507744 and includes the detection and sorting of non-polyamide 6 products, their crushing and the separation of dirt and other types of additives. For the separation of individual components such as they occur, for example, in the crushing of carpets in the form of foam backing, pile and supporting material, multi-stage floatation and sedimentation processes are preferably employed, followed by centrifuges and salt solutions of various densities to cause the lighter fraction, such as polypropylene fabric, to float and the heavy, filled carpet backing to form the sediment. A Polyamide 6 fraction obtained in this way is freed from salt by washing and is dried in a continuous process and then passed to depolymerisation via an extruder. In the depolymerisation, methods are followed, for example, according to those known from DE 887 199, EP 0875504, DE 910056, U.S. Pat. No. 4,605,762 and JP 53-13636, in which, using phosphoric acid, breaking down of the polymer chains takes place and where introduced steam drives off the produced caprolactam as a water-steam mixture.
Furthermore, a method for the manufacture of purified caprolactam from a carpet containing polyamides is known from U.S. Pat. No. 5,990,306 in which the caprolactam is distilled and crystallised after depolymerisation.
In addition, a method for the continuous recovery of caprolactam from polycaprolactam waste is known from U.S. Pat. No. 4,107,160, whereby, among others, a caprolactam-water mixture which is obtained through the distillation of the caprolactam, is mixed with fresh water vapour and passed to depolymerisation as a superheated mixture.
The obtaining of highly pure caprolactam from Polyamide 6 waste, such as from used carpets or the other products mentioned above, is continuously subject to variations in the composition of the flow of input material. Consequently, on one hand, the secondary constituent components of a carpet, such as foam backing or backing fabrics, vary depending on the manufacturer and on the other hand, the Polyamide 6 is differently mixed with additives and colorants, etc. Therefore, a non-uniform and continuous supply of raw materials prevails. To manufacture a highly pure product from a material with such extreme variation in the input products and which meets or exceeds the quality requirements of the synthesized caprolactam is an objective not yet achieved on a large scale.
A disadvantage with the known methods of processing Polyamide 6 waste is therefore that they do not supply any highly pure caprolactam which can be used for manufacturing Polyamide 6.